The present invention relates to a power converter apparatus, and in particular to a power converter apparatus linked to an AC power system via a transformer.
A document written by Makoto Hagiwara and Hirofumi Akagi and entitled “PWM Control and Experiment of Modular Multilevel Converters (MMC)” Journal of The Institute of Electrical Engineers of Japan, vol. 128, No. 7, 2008, pp. 957-965, has proposed a modular multilevel converter (MMC) by using switching devices (Insulated-Gate Bipolar Transistor, IGBT etc.) to be able to control on and off, as a system of a power converter apparatus capable of outputting a high voltage exceeded over a withstand voltage of the switching device.
The MMC is a converter using a bidirectional chopper circuit or full bridge circuit, as a unit cell, connected to a DC capacitor, and input and output terminals of that are connected in cascade. The MMC has an aspect that the phase of PWM control carrier wave of the unit cell is shifted by every unit cell to thereby inhibit an output voltage harmonic wave. The MMC has been known that it can be used as a system-cooperative voltage converter, such as a reactive power output device, an active power storage device, etc.
When the modular multilevel converter is connected to a power system, an excessive current or an inrush current is flown into the DC capacitor via diodes configuring the bidirectional chopper circuit or full bridge circuit of the unit cell if the DC capacitor in every unit cell is not charged up. In this countermeasure, the above-mentioned document discloses an initial charge system which charges the DC capacitors from the AC power system via a resistor, as a system of charging up the DC capacitor safely.